Trim fat from steak. Use your fingers to press black pepper onto both sides of steak. In a large skillet cook steak in hot oil over medium heat for 15-20 minutes or until desired doneness, turning once. Remove from skillet; keep warm.

Add onion to drippings in skillet. (Add more oil, if necessary.) Cook for 5-10 minutes or until onion is nearly tender, stirring occasionally. Stir in mustard; remove from heat.

Meanwhile, if using Italian flatbread, cut it into six wedges; split each wedge in half horizontally. If using sourdough slices, toast them, if desired. To serve, cut steak into bite-size strips. Top bottom halves of Italian flatbread wedges or six slices of bread with spinach, steak strips, roasted pepper strips, onion mixture, and remaining bread wedges or slices.